What is the dramatic significance of the play? How do the
witches add to the drama?

In
considering the "dramatic significance" of the play, we might think
of whether Macbeth is a tragic hero. Act 1 at first
tells us he is heroic; we quickly learns he has a fatal
flaw,ambition. But is he responsible for his fall &
does he learn from his mistake (part of Aristotle's definition of
a tragedy). We turn to the witches: do they dictate his
fate? Many argue that Macbeth would not have gotten into the mess
he did if the witches had not appeared to him in the first
place. Remember, they are never up to any good, but since
Banquo resists temptation, certainly Macbeth could have as
well. Second, does he learn from his mistake? In the final
scene of the play, he first speaks to MacDuff with hubris, but
right before he dies, Macbeth again speaks like the warrior he was
at the beginning of the play, telling MacDuff: “I will not
yield, / . . . Yet I will try the last." Although he certainly ends
the play bravely, but he doesn't admit he has done wrong or that
he is guilty of excessive ambition. To the extent that he does
not, he fails as a tragic hero, and that might be the “dramatic
significance” of the play.

I would say that to put it very reductively, the "dramatic
significance" is "be sure your sins will find you out."
Macbeth and Lady Macbeth are consumed with fear and guilt, both
propelled by their insatiable desire for power.

As for the witches, they are yet another warning for Macbeth
that his sins will not go undiscovered or unpunished. They
prophesy his fate. He does not want to believe them, but they
speak a bloody truth. The first witch tells him "to beware
Macduff." The second promises that "none of woman born shall
harm Macbeth." The third tells him that he will be safe until
"Birnam Wood moves against him."

The witches prophecies add to the drama because as an audience,
we know these things will come to pass, even if Macbeth and his
Lady choose to stick their heads in the sand. It is just a
matter of time until each of the prophecies come to pass.

According the the definition of climax that climax is the final and most exciting event in a series of events, I think that the climax in Macbeth occurs in the final act when Macduff corners and kill Macbeth.

The play Macbeth is based on the theme of courage, greed and the loss of a man's soul. It shows how absolute power can corrupt absolutely. The play revolves around Macbeth, a man who was once adored, admired and praised for his exceptional skills on the battlefield, but who succumbs to his insatiable greed and innate desires. His ambitions coupled with that of his wife's becomes the cause of his ultimate downfall, in the eys of his public and in his own eyes as well. Life, which was once full of meaning for him, becomes devoide of hope, joy and love. This we understand when he calls life an illusion and compares to it an actor who frets and worries his hour on stage, and then is not hear from ever again. The theory of the downfall of a hero, is explained effectively.

The witches on the oter hand, contribute heavily to the 'drama'. Their prophecies, accelerate the already-scheming and ambitious man in Macbeth and lead him to commit the horrifying crimes which ultimately, cause his downfall. Their appearances in the play are the darkest, gloomiest aspects and signify the power of evil being able to tempt man into his own downfall.